When I wanted to do some dual extrusion everything seemed to be fine. Model was sliced, gcode produced (which looked fine in Cura+Repetier) and x3g loded on printer. First layers first color printed fine, but then problems started. When the extruder switched it started with a big blob, extruding so much filament that the extruder started to miss steps.

A lot of searching later I found a post here on the forum, that Cura does not support dual extrusion on Makerbots. Why? Because Cura developers need feedback from users in order to know how the gcode should look. Well, I'll try to give my 50 cents here.

I've done some searching, checking and trials and found what I believe is what needs to be fixed for Cura to produce dual extrusion gcode for Makerbots and clones.

When Cura produces dual extrusion gcode the E-steps are all added up.

Example - each extruder extrudes 10mm then switches

Start: T0 extrudes 10mm -> E increases from 0 to 10

switch to T1

E-step is 10 and T1 continues 10mm more --> E is now 20

switch to T0

E-step is 20 and T0 continues 10mm more --> E is now 30

So, if extruder0 have extruded 10mm filament and a switch is done then extruder1 continues from 10mm and onward. This is exactly the key problem, and the reason I got blobs when printing.

Makerbots add up E-steps too - but separate for each extruder.

So, if extruder0 have extruded 10mm filament and a switch is done then extruder1 starts from 0 and extrudes continues from 10mm and onward.

Example - each extruder extrudes 10mm then switches

Start: T0 extrudes 10mm -> E increases from 0 to 10 for T0

switch to T1

E-step is 0 for T1 and T1 continues 10mm more --> E is now 10 for T1

switch to T0

E-step is 10 for T0 and T0 continues 10mm more --> E is now 20 for T0

The difference between the 2 E-step calculations adds up more and more and will quickly become huge.

So... if the 'Makerbot flavour' gcode is change to handle the E-step absolute value per extruder instead of as a total for both extruders, then I believe Cura would produce Makerbot dual extrusion friendly gcode.

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G92 does not work with sailfish 7.5 as a extruder reset at layer change. Actually it has the strange impact that the extruder does not change even though an extruder change command it right at the line before. Posts elsewhere on the web suggest the G92 was broken in Sailfish 6.xx and haven't worked properly since.